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June 29, 2017 | Page 2 File No. 0504-119-00 <br />a. Benzene, toluene, ethylbenzene, total xylenes (BTEX) and methyl tert-butyl ether (MTBE) using <br />EPA Method 8260C; <br />b. GRPH using Method NWTPH-Gx; and <br />c. DRPH and ORPH using Method NWTPH-Dx. <br />3. Measuring depth to groundwater and calculated relative groundwater elevations to estimate <br />groundwater flow direction and gradient. <br />4. Drumming and labeling investigation-derived waste (IDW). A qualified contractor will be retained to <br />profile and transport the IDW for disposal at a permitted facility. We assume that IDW disposal for the <br />site will be completed in a single “milk run” at the conclusion of the site assessments. <br />5. Entering the analytical data into Ecology’s Environmental Information Management (EIM) database. <br />6. Preparing this monitoring report. <br />4.0 FIELD ACTIVITIES <br />4.1. Groundwater Sampling <br />Groundwater samples were collected from monitoring wells (MW-1 through MW-6) on May 12, 2017. Depth <br />to groundwater, measured from the top of the PVC well casings, ranged from approximately 3.76 feet <br />(MW-2) to 4.95 feet (MW-6). Groundwater elevations ranged from approximately 1569.56 feet in <br />monitoring well MW-5 to 1573.68 feet in monitoring well MW-1. Depth to water and groundwater elevations <br />are presented in Table 1 below. <br />Monitoring wells MW-1 through MW-6 were purged and sampled using low-flow sampling techniques. <br />A peristaltic pump equipped with disposable tubing was used to purge and sample each well. Groundwater <br />quality parameters generally were measured at 3-minute intervals during well purging. Groundwater <br />samples were collected when each water quality parameter had stabilized in conformance with the criteria <br />presented in Appendix A or a maximum purge time of 30 minutes was achieved. Field methods are <br />described in Appendix A. A duplicate sample was collected from monitoring well MW-3. <br />Based on groundwater elevations measured on May 12, 2017, groundwater flow generally was toward the <br />southwest, as depicted in Figure 2. Excluding data from well MW-5, the estimated hydraulic groundwater <br />gradient across the site was about 0.005 feet per foot (which is approximately 26 feet per mile). <br />Groundwater elevations decrease toward the southwest near MW-5. Including data from well MW-5, the <br />estimated groundwater gradient is 0.0118 feet per foot, which is approximately 62.5 feet per mile. <br />Groundwater elevation contours provided on Figure 2 were based on depth to water measurements, <br />surveyed elevations of well casings, Surfer Version 1.2 modeling, and our hydrogeological interpretation. <br />Relatively low DO and ORP levels at MW-2, within the former UST basin indicate the potential reducing <br />conditions that support bioremediation. The ORP levels in the upgradient well (MW-1) and downgradient <br />wells (MW-5 and MW-6), are elevated; as is ORP in MW-3 which is closer but cross-gradient to the former <br />UST area. DO is high in MW-1 and MW-6, moderately high in MW-3 and low in MW-4 and MW-5, possibly <br />indicating an anaerobic zone extending from MW-2 to MW-5.